Mechanisms located in the cell membrane on the apical side of the epithelial cells will be investigated in order to gain more basic information about the regulation of ion movement across epithelial tissues. A large body of evidence indicates that the apical cell membrane plays a central role in the regulation of Na transport across epithelial cells and very recent experiments show that a similar role must be assigned to the apical cell membrane for Cl transport across the cells. Yet very little is known about the mechanisms by which this regulation occurs as direct data are lacking. It is planned therefore to get direct and specific information about the kinetics of Cl and Na movement in both directions across the apical cell membrane under conditions which permit the investigation of the molecular mechanisms involved in these regulatory processes. The changes in apical cell membrane function will be studied, for example, when: (a) the composition of the solution in contact with the epithelial cells is altered by ionic substitution or change in pH; (b) the membrane potential is changed by voltage clamping at different potentials; (c) as a result of hyperpolarization, the Cl exchange diffusion system in the apical cell membrane is replaced by a conductive (passive) Cl transport system and (d) when, as a result of Fe ion application to the apical cell membrane there is a transition from active Na transport to active Cl transport. The kinetics of the Cl and Na transport systems will be investigated by combining two techniques: (1) direct measurements of tracer fluxes across the apical cell membrane, alone and together with measurements of transepithelial fluxes, and (2) determinations done with intracellular microelectrodes which will provide information about the electrical properties of the cell membrane and the potential gradients across cell membranes as well as about intracellular ion activity (Cl, later in project possibly also Na and K). The project will be done on tight Na transporting epithelia (frog skin, toad skin, urinary bladders of Amphiuma or Necturus), mounted in chambers (1) as isolated tissues and (2) as preparations of epithelial cell layers with connective tissue. The main goal of this project is the achievement of a better understanding of the epithelial transport processes. These processes constitute the most important function of many vital organs of the body.